Locations in presubmission checklist

[drpaulralph]

1. As an author, I want to indicate the location of each presubmission checklist item (instead of just indicating that it's in the paper somewhere), to prevent reviewers from missing things.
2. As an author, I want to download a txt export of the presubmission checklist, which shows the locations of each item, so that I can provide it to reviewers.

Notes:

• "location" does not apply to all entries. For example, "contributes in some way to the collective body of knowledge" wouldn't have a location
• Some items may have multiple locations. It should be easy for the author to say indicate multiple locations
• The best way to do this would be with line numbers but not all docs have line numbers so we need to be flexible
• I'm not sure what this should look like exactly.